Because he's fused with Tony's soul, his mental defenses are much tougher then ordinary people since he's technically two at the same time. Not to mention that MCU Tony should have a good mental defense considering the company he keeps like Strange and the Scarlet Witch. Kind of like how Batman can block things like that using sheer willpower.

Sorry, this took a while to get back to. Same answer as another comment, the original arc reactor was built using materials provided by Ten Rings to form the Jericho missile. He doesn't have access to those, though he will soon. Thank you for your comment.

The reason Tony was able to build an arc reactor in the cave was because of the materials provided by Ten rings to build the Jericho missile. MC doesn't have those resources to use.

No, he did not create the arc reactor. It wouldn't make sense if he had one, since he's just a teenager from Gotham. Instead, he uses an array of batteries located in his chest plate, just like in the comics. He's not Tony Stark, just his normal DC counterpart that inherited his memories. Also, the soul fragment comes from the MCU not the armored adventures.

He was smart but not as smart as Tony was at his age. So he was a genius, but he just became smarter.

This is Marvel Cinematic Universe Tony, because the movies are much less complicated then the comics. Thanks for asking!